The present invention relates generally to semiconductor manufacturing and process control, and specifically to measurements of critical dimensions of semiconductor device features.
When microelectronic devices are produced on a semiconductor wafer, it is crucial that the critical dimensions of the devices be held within specified tolerances. Critical dimensions, in this context, refer to the widths of features, such as conductors, that are deposited on the wafer and the spacing between adjacent features. Deviations from the specified dimensions lead to performance and yield degradation. The manufacturing process must therefore be carefully monitored and controlled, in order to detect deviations as soon as they occur and to take corrective action to avoid the loss of costly wafers in process. For example, when a critical dimension in photoresist that has been deposited and etched on the wafer is found to be out of specification, it is possible to remove and reapply it.
A variety of systems and methods for measurement of critical dimensions are known in the art. Most microelectronic production facilities currently use optical metrology to monitor critical dimensions. As semiconductor devices become ever denser, however, with design rules of 0.25 xcexcm and below, it becomes impossible for classical optical metrology systems to provide accurate results. Electron beam (e-beam) metrology has been suggested as an alternative, but e-beam systems also suffer from performance limitations.
A further problem in critical dimension measurements is the high aspect ratio and non-uniform width of features created on the semiconductor wafer. For example, in order to produce vias, a layer of photoresist is deposited on the wafer surface. The photoresist is exposed to ultraviolet radiation, hardened and then etched to form trenches, which are subsequently filled with metal. The photoresist is typically 0.7 to 1.2 xcexcm thick, while the trenches are only 0.1 to 0.2 xcexcm wide. (This width is the critical dimension of the vias.) Because of the etching process, the walls of the trenches tend to slope inward. The trenches are therefore wider at the top and narrower at the bottom, where they encounter the underlying wafer surface below the photoresist. It is the smaller width at the bottom of the trench that is most critical. Optical and e-beam metrology are not well suited for measuring this dimension, however, because of the high aspect ratio of the trenches.
X-ray reflectance and fluorescence measurements are commonly used to determine the thickness and composition of thin film layers, including particularly metal layers formed on semiconductor wafers. For example, U.S. Pat. No. 5,740,226, to Komiya et al., and U.S. Pat. No. 5,619,548, to Koppel, whose disclosures are incorporated herein by reference, describe film thickness measurements based on X-ray reflectometry.
Another method for thin film measurement using X-rays is described by Hayashi et al., in an article entitled xe2x80x9cRefracted X-rays Propagating near the Surface under Grazing Incidence Condition,xe2x80x9d published in Spectrochimica Acta, Part B 54 (1999), pages 227-230, which is also incorporated herein by reference. The authors irradiated a silicon wafer having an organic thin film coating with X-rays incident at a grazing angle. They measured the energy of the X-rays that propagated along the surface of the wafer and discovered two peaks: one corresponding to refraction at the upper boundary between the thin film coating and the ambient air, and the other to refraction at the boundary between the coating and the wafer substrate. The energies of these peaks correspond to the critical energies of the respective boundaries. Below the critical energy, the X-rays are totally reflected from the boundary, while above the critical energy the X-rays pass through the boundary and are refracted. The critical energy for any given boundary depends on the angle of incidence of the X-rays and on the refractive indices of the materials separated by the boundary.
The methods of X-ray measurement mentioned above, as well as all other known X-ray methods, are limited to measurement of thickness or depth, i.e., of dimensions measured perpendicular to the plane of the wafer substrate. X-ray techniques known in the art do not generally have sufficient spatial resolution for use in measuring critical dimensions of feature width, i.e., dimensions of the features in a direction parallel to the substrate plane.
It is an object of some aspects of the present invention to provide improved methods and apparatus for measurement of critical dimensions of microelectronic devices, particularly during stages of wafer fabrication.
It is a further object of some aspects of the present invention to provide methods and apparatus capable of measuring the dimensions of microscopic features on a semiconductor wafer or other substrate having a high aspect ratio of height to width.
In preferred embodiments of the present invention, X-ray scattering from features on the surface of a substrate is detected in order to determine dimensions of the features, and particularly to measure the width of such features. Typically, the substrate comprises a semiconductor wafer, on which a test pattern is formed for the purpose of measuring critical dimensions of functional features of microelectronic devices in fabrication on the wafer. Preferably, the test pattern comprises a grating structure, made up of a periodic pattern of ridges, having attributes (such as height, width and spacing) similar to those of the functional features in question. When an X-ray beam irradiates the pattern, the resultant scattered radiation has a spatial modulation that is characteristic of the critical dimensions. A detection system senses and analyzes the modulation of the scattered radiation in order to determine the critical dimensions.
In some preferred embodiments of the present invention, the scattered radiation received by the detection system comprises reflected radiation. Preferably, the irradiating beam is well collimated and is incident on the surface at a grazing angle, nearly parallel to the surface of the substrate. Most preferably, the beam is incident in a direction whose projection on the surface is parallel to the ridges of the pattern. As a result, spatial modulation in the form of a xe2x80x9cshadowxe2x80x9d of the test pattern is imposed on the reflected radiation. The critical dimensions of the pattern are determined based on the characteristics of the modulation.
In one of these preferred embodiments, a collimator, which is used to collimate the irradiating beam, itself comprises a grating made of a pattern of ridges formed on a base. Preferably, the ridges on the collimator comprise metal ridges, having a period substantially equal to the period of the test pattern on the substrate under test. Reflection of the X-ray beam from the collimator at a grazing angle both collimates the beam and imposes on it the periodic pattern of the ridges. When the collimated beam is incident on the test substrate, an interference pattern is formed between the pattern imposed by the collimator and the test pattern. The detection system senses the interference pattern in order to determine the critical dimensions of the test pattern. Most preferably, the test substrate is translated relative to the collimator (or vice versa), causing the interference pattern to vary. The detection system senses this variation, which is then analyzed to determine the critical dimensions.
In other preferred embodiments of the present invention, the detection system captures a portion of the scattered radiation that is refracted at the surface of the substrate at an angle that is roughly parallel to the surface. The detection system analyzes the energy of X-ray photons that it captures and thus determines the critical energies for total external reflection at the surface, in a manner similar to that described in the above-mentioned article by Hayashi et al. Two different peaks are detected in the energy spectrum: one for the boundary between the ridges of the test pattern and the substrate, and the other for the boundary between the substrate and the ambient air, in the space between the ridges. The detection system analyzes the two peaks to determine the average width of the ridges relative to the intervening spaces.
Although preferred embodiments are described herein with reference to measurements of critical dimensions of features on semiconductor wafers, the principles of the present invention are similarly applicable to measuring the dimensions of microscopic features formed on substrates of other types. Furthermore, although these preferred embodiments are described with reference to particular types of test patterns and measurement modes, other methods of measuring critical width dimensions using X-rays will be apparent to those skilled in the art upon reading this description. All such methods are considered to be within the scope of the present invention.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for measurement of critical dimensions, including:
irradiating a surface of a substrate with a beam of X-rays;
detecting a pattern of the X-rays scattered from the surface due to features formed on the surface; and
analyzing the pattern to measure a dimension of the features in a direction parallel to the surface.
Preferably, irradiating the surface includes collimating the beam of X-rays that is to irradiate the surface.
Further preferably, the features formed on the surface include a periodic structure on the substrate having a predetermined period, and collimating the beam of X-rays includes imposing on the beam a spatial modulation, and detecting the pattern includes observing an interference pattern generated due to interaction of the spatial modulation with the periodic structure. Most preferably, imposing the spatial modulation includes modulating the beam with a spatial period substantially equal to the predetermined period of the structure. Additionally or alternatively, the periodic structure on the substrate includes a first grating structure, and imposing the spatial modulation includes reflecting the beam from a second grating structure, wherein analyzing the pattern includes observing changes in the interference pattern as one of the first and second grating structures is translated relative to the other.
Additionally or alternatively, the features include a periodic structure, and wherein detecting the pattern includes detecting a modulation of the reflected X-rays due to the periodic structure. Preferably, the periodic structure includes a grating made up of a plurality of ridges oriented parallel to a grating axis, and irradiating the surface includes directing the beam of X-rays toward the surface such that a projection of the beam on the surface is substantially parallel to the grating axis. Further additionally or alternatively, the periodic structure includes a grating made up of a plurality of ridges oriented parallel to a grating axis, and irradiating the surface includes directing the beam of X-rays toward the surface such that a projection of the beam on the surface is substantially perpendicular to the grating axis.
Still further additionally or alternatively, the periodic structure includes a thin film layer formed on the substrate and etched to form the periodic structure. Preferably, the thin film layer is etched to form ridges while exposing an area of the surface of the substrate between the ridges, and detecting the pattern includes detecting variations in the scattered X-rays indicative of a width of the exposed area.
Preferably, detecting the pattern of the X-rays includes detecting a variation in the scattered X-rays associated with total external reflection from an area of the surface. Further preferably, the substrate includes a first material having a first refractive index, and wherein the features include a second material, having a second refractive index different from the first refractive index, and detecting the variation includes finding a difference between the total external reflection of X-rays incident on a boundary between the first and second materials and of X-rays incident on the first material in an area of the surface on which the second material is substantially absent. More preferably, irradiating the surface includes irradiating the surface near a critical angle of the first material, such that the beam of X-rays is reflected from the area of the surface on which the second material is substantially absent, but is substantially not reflected from the boundary between the first and second materials. Most preferably, finding the difference includes detecting a spatial modulation of X-rays reflected from the surface due to the total external reflection.
Additionally or alternatively, finding the difference includes detecting a distinction between the critical energies for total external reflection from the boundary between the first and second materials and from the area of the surface on which the second material is substantially absent. Preferably, detecting the distinction includes detecting X-rays refracted parallel to the surface, and analyzing the pattern includes analyzing the detected X-rays to find the critical energies.
Still further additionally or alternatively, detecting the pattern of the X-rays includes detecting a variation in refraction of X-rays at the surface.
Preferably, the substrate includes a semiconductor wafer, and the features formed on the surface include photoresist deposited on the surface.
There is also provided, in accordance with a preferred embodiment of the present invention, apparatus for measurement of critical dimensions, including:
an X-ray source, configured to irradiate a surface of a substrate with a beam of X-rays;
a detector, adapted to detect a pattern of the X-rays scattered from the surface due to features formed on the surface and to generate a signal responsive to the pattern; and
a processor, coupled to receive and analyze the signal so as to measure a dimension of the feature in a direction parallel to the surface.
Preferably, the X-ray source includes a collimator, adapted to collimate the beam of X-rays that is to irradiate the surface.
In a preferred embodiment, the apparatus includes a translation device, adapted to shift one of the first and second gratings relative to the other, wherein the processor is adapted to analyze changes in the interference pattern as the one of the first and second gratings is shifted.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a collimator, including:
a substrate; and
a grating structure formed on the substrate, the structure including a plurality of parallel ridges aligned parallel to an axis of the grating, so as to collimate a beam of X-rays incident on the substrate at a grazing angle.
Preferably, the substrate includes a first material, which reflects the X-rays incident at the grazing angle, and wherein the ridges include a second material, which absorbs the X-rays. Most preferably, the first material includes a semiconductor material, and the second material includes a metal.